When one accidently drops food, not held in a container or wrapper, on the ground. The rule states that if you pick it up within 5 seconds, it won't be contaminated. There is also a '10 second rule' variation. Unfortunately, both rules are urban legends -- The speed with which you can pick the food article back up has no bearing on whether it may receive germs.

The rule that applies to lost or fallen objects, typically food, that hits the floor. The rule applies to general areas, and does not apply to anything in a bathroom. Using this rule makes it OK to pick up said object in under 5 seconds, under the presumption that no germs attached themselves in such a short period of time.

If your gum just dropped on the floor, pick it up and say "5 Second Rule", then place it back in your mouth.